mesoporosity for robust supercapacitor with superior volumetric capacitance and cyclic performance

mesoporosity for robust supercapacitor with superior volumetric capacitance and cyclic performance

Accepted Manuscript Nanosized graphitic carbon with balanced micro/mesoporosity for robust supercapacitor with superior volumetric capacitance and cyc...

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Accepted Manuscript Nanosized graphitic carbon with balanced micro/mesoporosity for robust supercapacitor with superior volumetric capacitance and cyclic performance Chunfeng Xue, Fujuan Yang, Enyang Wang, Lin Feng, Xiao Du, Xiaogang Hao, Xupeng Li PII:

S0013-4686(18)30689-3

DOI:

10.1016/j.electacta.2018.03.172

Reference:

EA 31538

To appear in:

Electrochimica Acta

Received Date: 4 December 2017 Revised Date:

22 March 2018

Accepted Date: 26 March 2018

Please cite this article as: C. Xue, F. Yang, E. Wang, L. Feng, X. Du, X. Hao, X. Li, Nanosized graphitic carbon with balanced micro/mesoporosity for robust supercapacitor with superior volumetric capacitance and cyclic performance, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.03.172. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphic Abstract

Nanosized graphitic carbon with balanced micro/mesoporosity for robust

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supercapacitor with superior volumetric capacitance and cyclic performance

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Chunfeng Xue,a* Fujuan Yang,a Enyang Wang,a Lin Feng,a Xiao Du,a Xiaogang Hao*a and Xupeng Li b

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Ultrahigh volumetric capacitance and energy density plus excellent cyclic stability perform in the symmetric supercapacitors based on carbons with intercalated micro/mesopores.

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Nanosized graphitic carbon with balanced micro/mesoporosity for robust supercapacitor with superior volumetric capacitance and cyclic performance

Xiaogang Hao*a and Xupeng Lib

College of Chemistry and Chemical Engineering, Taiyuan University of

Technology, Taiyuan, 030024, P. R. China

School of Chemistry and Material Science, Shanxi Normal University,

Linfen, 041004, P. R. China.

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b

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a

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Chunfeng Xue,a* Fujuan Yang,a Enyang Wang,a Lin Feng,a Xiao Du,a

Abstract: A nanosized graphitic carbon with balanced and connected micro/mesopores is prepared by directly carbonizing Ni(II) cation-exchanged

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resin at a moderate temperature of 800 oC. The homogeneously dispersed Ni nanoparticle well acts as the catalyst for “in-situ” graphitization of the low-cost

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resin and the porogen for the formation of configured micro- and mesopores. The graphitic carbon GC-800 exhibits a balanced micro/mesoporosity at the

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ratio of 1:2, which delivers an excellent specific capacitance 66.0 µF/cm2 and a high volumetric capacitance of 552.6 F/cm3 at the current density of 0.50 A/g in the electrolyte of 1.0 M H2SO4. In the dual-electrode system, it also displays high energy density of 7.43 Wh/kg at a power density of 125.0 W/kg. Its superior cyclic performance is not only confirmed by the capacitance retention (95%) after 400 h using the rigid floating technique but also 10000 cycles of GCD measurement at the current density of 5.0 A/g in both double- and 1

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triple-electrode systems. Keywords: Supercapacitor; Cyclic performance; Volumetric capacitance; Balanced porosity; Graphitic carbon

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1. Introduction With the worsening energy crisis and environmental pollution, the efficient and eco-friendly resources have attracted widespread attention. Electric

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double-layer capacitors (EDLCs) possess wide voltage window, fast

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charge-discharge rate and high power density, which are good candidates used as energy storage devices [1, 2]. Since their energy store in the double layers at the electrode-electrolyte interface [3], many porous carbons including graphene[4, 5], carbon nanotube [6], activated carbon[7] and hydrothermal carbon[8] are

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selected as the electrode materials for assembling supercapacitors[9]. As known, it is reported that carbons with high specific surface area benefit to form electric double-layer in the electrodes. For example, mesoporous carbon

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sphere array with specific surface area of 601 m2/g displays a specific

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capacitance of 14.0 µF/cm2 (84 F/g) [10]. Amorphous carbon CMK-3 shows a specific surface area of 900 m2/g and a specific capacitance of 10 µF/cm2 (90 F/g) [11]. The CuO activated mesoporous carbon with specific surface area of 1084 m2/g exhibits a specific capacitance of 7.2 µF/cm2 (78 F/g) [12]. A mesostructured carbon with attractive specific surface area of 2390 m2/g delivers a specific capacitance of 4.7 µF/cm2 (112 F/g) [13], which is similar to the activated carbon. However, it seems that no obvious enhancement in the specific 2

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capacitance derives from the improved specific surface area of the carbons. The obtained specific capacitances are far below the theoretical value (about 20 µF/cm2) [14]. Therefore, the high specific surface area is not the exclusive factor

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for the carbon to get ideal specific capacitance. To be a promising electrode material, micropore and mesopore of carbons should not only match in their volume but also effectively intercalate together

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[15-17]. For the microporous carbons with high specific surface area, the

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micropores in the micrometer-scale grain are too long and narrow for the electrolyte ion to pass in a short time [15]. Alternatively, for the carbons with moderate specific surface area exclusively deriving from mesopores, the ion transfer rate is ten times faster than that in the micropores [18]. However, the

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solvating effect and less electric dual-layer may inhibit the final specific capacitance. Feasibly, hierarchically porous structure in the carbon benefits to fabricate desired electrodes [16, 19-21]. As known, the activated carbon with

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specific surface area up to 2000.0 m2/g is considered to benefit to the charge

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storage in EDLCs [10]. It covers micropore, mesopore, and even macropore in which an arbitrary pore interconnection exists. This kind of pore configuration actually contains blind alley and transfer faultage, which seriously inhibits the continuous and fast mass transfer. Thus, it is desired to match and connect the micro- and mesopore in the carbon to achieve an attractive specific capacitance by improving the electrolyte ion transfer [22]. Besides the tortuous micropores in activated carbons, their amorphous 3

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texture with high ion-transfer resistance may result in poor specific capacitance [23]. To achieve better electrochemical performance, graphitic carbons have been developed by directly heating the amorphous carbon (a-C) at about 2500 C or “in-situ” catalytically graphitizing it at moderate temperature. High

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temperature carbonization is a traditional means, which obviously needs huge energy consumption and complex experiment device. Unfortunately, it is hardly

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effective for non-graphitized carbon and even ruins the pristine pore structure[24]. Alternatively, catalytic graphitization with the assistance of

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transition-metal at around 1000 oC is an effective route to produce graphitic carbon [25-27]. For example, the graphitized mesoporous carbons using the catalyst of metal Fe, Ni or Mn surely exhibit low ion-transfer resistance [28-30].

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Graphitic carbon catalyzed by Cr shows a specific capacitance of 122.5 F/g at the current density of 1.0 A/g [26]. Graphitic porous carbon derived from the resin containing Mo7O246- ions plus H2O2 oxidation treatment shows a specific

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capacitance of 265.0 F/g in the electrolyte of 6.0 M KOH [25]. Carbon

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nanosphere from a simultaneous activation and graphitization of resin immersed with ZnCl2 and FeCl3 displays specific capacitance of 24.17 µF/cm2 in the triple-electrode cell [31]. The carbons mentioned above display improved but different specific capacitance partially contributing from their graphitic texture. All the factors mentioned above can inhibit ion transfer, electrolyte accessibility and electron transport, and finally limit the specific capacitance. Therefore, it is interesting to mediate high specific surface area, matched micro/mesopore 4

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structure, and good electric conductivity into the carbon for assembling attractive electrodes of supercapacitors. Recently, it is reported that Ni nanoparticle is a good catalyst for the

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spontaneous graphitization of the a-C even at room temperature [32]. We noted that mechanically mixed Ni species with the a-C or adsorbed onto the carbon precursor often result in fragmented graphitization domains in the resultant

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samples [33]. From our viewpoint, good catalytic activation of metal species and

graphitic

carbon. Here,

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their monodispersion in the a-C substrate is vital important in preparing using

“in-situ”

carbonization

of

resin

with

homogeneously dispersed Ni2+, we prepared graphitic carbon with balanced and connected micro/mesopore. The whole strategy is simple just as proposed in

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Scheme 1, the commercial cation-exchange resin (Tulson CH-90) with monodispersed Na+, cross-linked polystyrene containing iminodiacetic group, is used as the carbon precursor. It is firstly soaked in HCl solution and turns into

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protonated resin (H-CH-90). Then the sample H-CH-90 exchanges with

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Ni(NO3)2 solution. Subsequently, the resin with highly dispersed Ni2+ (Ni-CH-90) is carbonized at 800 oC and converted into Ni nanoparticle containing graphitic carbon (Ni-GC-800). During the process, the Ni species catalyze the graphitization of the a-C in their vicinity and the formation of micropores. Concomitantly, the Ni species may aggregate into nanoparticles and further create the mesopores, which well balance in volume and connect with each other. Finally, graphitic carbon with balanced micro/mesoporous structure 5

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is prepared after the removal of Ni nanoparticle. The obtained carbon (GC-800) demonstrates a significantly specific capacitance of 66.0 µF/cm2 and volumetric capacitance of 552.6 F/cm3 at the current density of 0.5 A/g and an exciting

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cyclic stability even after 400 h test via the floating technique. 2. Experimental Section 2.1 Materials

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Polystyrene-based cation-exchange resin (Tulsion CH-90) was purchased from Cohesion (Beijing) Co. Ltd. Graphite paper was purchased from Beijing

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Jinglong Special Carbon Technology Co. Ltd. Nickel nitrate (Ni(NO3)2·6H2O), anhydrous ethanol (EtOH, >99.99%), sulfuric acid (H2SO4, 95.0% ‒ 98.0%) and N, N-dimethyl formamide (DMF) were bought from Tianjin Fuchen Chemical

lab.

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Reagent Co. Ltd. Ultrapure water (Millipore 18.2 MΩ·cm) was prepared in the

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2.2 Preparation of graphitic porous carbons First, Tulsion CH-90 was treated with 5.0% HCl solution and washed with

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the ultrapure water until the eluate become neutral. Subsequently, the protonated resin (H-CH-90) was dried under vacuum at 60.0 oC for 6.0 h. Then, the dried H-CH-90 was soaked in 0.10 M Ni(NO3)2 solution and stirred for 24.0 h in order to exchange completely. After that, the exchanged resin (Ni-CH-90) was washed with deionized water and dried at 110.0 oC for 6.0 h. Then the sample Ni-CH-90 was carbonized at 800.0 oC in N2 with a purity of 99.99% for 1.0 h. Subsequently, the resultant sample Ni-GC-800 was ground at a speed of 600.0 6

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rpm in the ball milling for 2.0 h and washed with 3.0 M HCl solution at 150.0 oC for 48.0 h to remove the Ni species. It is centrifuged and washed using ultrapure water for several times in order to remove the residue. Finally, the graphitized

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carbon was dried at 110.0 oC for 24.0 h and denoted as GC-800. For comparison, the samples GC-700 and GC-900 are also prepared following the above procedure but carbonized at 700.0 and 900.0 oC separately. Additionally, the

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untreated resin CH-90 was directly carbonized at 800.0 oC to obtain pristine

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carbon (PC-800) as a reference sample. The heating ramp is 5 oC/min. 2.3 Characterizations

X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV type X-ray diffractometer equipped with Cu Ka radiation (λ = 0.15406 nm, 40.0

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kV, 40.0 mA). Raman spectrum was recorded on the laser confocal Raman spectroscopy LabRAM HR800. Morphologies of samples were observed on a scanning electronic microscope LEO438VP. N2 adsorption-desorption isotherms

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were measured on a specific surface area and pore size analyzer JW-BK122W at

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‒196.0 oC. Before the measurement, the samples were degassed at 300 oC for 3.0 h. Specific surface area was calculated using Brunauer-Emmett-Teller (BET) method. Pore size distribution was calculated from adsorption branch of the isotherms according to Barrett-Joyner-Halenda (BJH) model. CO2 and N2 adsorption isotherms at 0 oC of the sample PC-800 were also measured on the analyzer JW-BK122W. Before the measurement, the sample was degassed at 300.0 oC for 8.0 h. Thermogravimetry (TG) curves were collected on the 7

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instrument Netzsch STA 449 F5 TG/DTG. About 10.0 mg sample was placed into a crucible and heated from 30.0 to 900.0 oC with a rate of 5.0 oC/min in air. 2.4 Electrochemical measurement

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All electrochemical experiments in this work were performed using a VMP2 electrochemical workstation (Princeton, USA). In a triple-electrode system, a piece of platinum foil and a saturated calomel electrode were used as

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counter electrode (CE) and reference electrode (RE), respectively. Working

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electrode (WE) was prepared by coating a slurry composed of 80.0% carbon, 10.0% acetylene black, and 10.0% poly (vinylidene fluoride) binder in DMF on the graphite paper (effective area is 1.0 cm2) and dried under vacuum at 60.0 oC for 6.0 h. Cyclic voltammetry (CV) curves in the electrolyte 1.0 M H2SO4 were

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investigated in the potential range from ‒0.20 V to 0.80 V with the scan rates from 5.0 to 200.0 mV/s. The galvanostatic charge/discharge (GCD) was conducted at the current densities of 0.5 ‒ 20.0 A/g over the same voltage range.

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The electrochemical impedance spectroscopy (EIS) was measured in the

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frequency ranging from 0.010 to 10000.0 Hz with a potential amplitude of 5.0 mV. In a double-electrode system, two nearly identical WEs were prepared using the above method, respectively. They were separated by a piece of filter paper soaked with the electrolyte 1.0 M H2SO4 and assembled as the symmetrical supercapacitor. The mass of carbon material loaded in each electrode was about 1.0 mg. In the triple-electrode system, the specific capacitance for the WE was 8

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calculated according to the CV curve using the equation (1): C =

∫ IdV 2m × ∆V × v

(F

/ g)

(1)

The specific capacitance of the electrode at various current densities was

C=

I∆ t (F / g ) m∆V

(2)

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measured by the GCD method and calculated according to the equation (2):

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Where ∫ IdV (W) is the integrated area of the CV curve, I (A) is the constant current, m (g) is the mass of activated material, ∆V (V) is the potential window,

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v (mV/s) is the scan rate, and ∆t (s) is the discharge time.

In the double-electrode system, the specific capacitance (Csp, F/g) was measured by the GCD method and calculated depending on the equation (3): C sp =

4 I∆ t (F / g ) m ∆V

(3)

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Where I (A) is the constant current, ∆t (s) is the discharge time, m (g) is the total mass of the two carbon electrodes, and ∆V (V) is the range of potential.

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The energy density (E) and power density (P) were calculated depending on the equation (4) and (5), respectively: C ∆V E = sp (Wh / kg) 8× 3.6

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2

P=

(4)

E (W / kg ) ∆t

(5)

Where Csp (F/g) is the specific capacitance in the double-electrode system,

∆V (V) is the range of potential change, and ∆t (s) is the discharge time. 3. Results and Discussion 3.1 Morphology observation 9

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The macroscopic morphology images of samples PC-800 and GC-800 are shown in Fig. 1. The sample PC-800 looks irregular particle with arbitrary size as shown in Fig. 1 (A). Part of them is about 20 µm long. After careful

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observation, some macropores can be found on its external surface (Fig. 1 (B)), implying a remained pore structure of the resin Tulsion CH-90 during the carbonization process. The morphology of the sample GC-800 is different from

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that of PC-800 (Fig. 1 (C)). The particles of the sample GC-800 are smaller than

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those of the sample PC-800 (Fig. 1 (D)). Most of them are agglomerates of nanoparticles with 50 ~ 100 nm in diameter (Fig. 1 (D) and (E)). Some are isolated nanoparticles with similar diameter (Fig. 1 (F)). The results on the dimensional changes may contribute from catalytic pyrolyzation, thermal

3.2 XRD study

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shrinkage, and ball-milling treatment.

XRD patterns of samples are piled for careful comparison in Fig. 2. Two

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broad diffraction peaks at about 2θ = 23.2° and 43.5° can be observed for the

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sample PC-800 (Fig. 2 (A)), indicating an a-C texture [12, 31]. The results imply the carbonization temperature of 800 °C is not high enough to graphitize the resin CH-90. For the sample Ni-GC-800, three sharp diffraction peaks at 2θ = 44.5°, 51.8°, and 76.3° are observed in its pattern (Fig. 2 (B)), corresponding to (111), (200), and (220) reflections of metallic Ni (JCPDS 04-0850), respectively [27]. The results suggest that the exchanged Ni2+ has been reduced during the carbonization process. After the sample Ni-GC-800 is washed using the HCl 10

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solution, the obtained sample GC-800 exhibits a peak at 2θ = 26.1° in its XRD pattern (Fig. 2 (C)), assigning to the (002) plane of graphitic carbon [34]. The result demonstrates that the graphitic texture of the sample GC-800 is produced

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with the assistance of Ni nanoparticle at the lower temperature (800 °C) than the previous results [29, 35]. 3.3 Raman spectra analysis

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Raman spectra of samples PC-800 and GC-800 are collected to further investigate their graphitic texture (Fig. 3). We can find two peaks distributed to

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the disorder (D) band (~1347 cm-1) and the graphite (G) band (~1580 cm-1) (Fig. 3 (A) and (B)), respectively [36]. As we know, graphitization degree of carbon materials can be evaluated by calculating their intensity ratio of the G-band to

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D-band (IG/ID), namely, the higher value of IG/ID the better graphitization degree[10, 37]. More accurately, the IG/ID values of the samples PC-800 and GC-800 herein are calculated at about 0.574 and 0.832 basing on their peak area

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separately. Obviously, the graphitization degree of the sample GC-800 is higher

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than that of PC-800. Noticeably, a well-resolved peak emerges around 2690 cm-1 for the sample GC-800 in Fig. 3 (B), historically named G′-band, which is the second typical peak always observed in highly graphitic material [31]. Combining the results with its XRD pattern (Fig. 2 (C)), we further confirm the graphitic texture of the sample GC-800 [38]. 3.4 TG Analysis TG and differential thermogravimetry (DTG) analysis of the samples PC-800, 11

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GC-800, and Ni-GC-800 can reveal the difference on their thermal stability tied with graphitization degree. As can be seen in Fig. 4 (A)a and (B)a, the weight loss of the sample PC-80 mainly occurs at 450 ‒ 620 °C, corresponding to the

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decomposition of the a-C [34]. Its total weight loss is about 98.4%, implying a fully amorphous texture. In case of the sample GC-800, besides a small weight loss of 2.0% below 100 °C, two obvious weight losses of 81.7% and 13.8% are

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observed in two ranges of 450 ‒ 520 °C and 520 ‒ 640 °C (Fig. 4 (A)b, and

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(B)b), which correspond to the decomposition of the a-C and graphitic carbon, respectively [34]. The total weight loss of 97.5% indicates that the metallic Ni is almost removed from the carbon. Based on the results, the percentage of graphitic carbon in the sample GC-800 can be simply estimated to be 14.5% [17,

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34]. It is reasonable that the a-C displays lower thermal stability than the graphitic carbon in flowing air. It should be pointed out that the onset combustion temperature of the a-C is lower than that of the sample PC-800. The

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result may contribute from their different porosity revealed by following N2

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adsorption isotherms (Fig. 5). The high porosity of the sample GC-800 is beneficial to the collision of air with carbon surface and result in a low onset combustion temperature. Similarly, the low porosity of PC-800 may lead to a high one. In addition, the sample Ni-GC-800 shows two weight losses of 43.0% and 20.0% at 300 ‒ 420 °C and 420 ‒ 500 °C (Fig. 4(A)c, and (B)c) respectively, ascribing to the catalytic decomposition of the a-C and graphitic carbon with 37% NiO species (corresponding to 29.9 wt% of metallic Ni) [17]. 12

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3.5 N2 adsorption isotherms The carbonization process also imposes significant effect on the pore structure of the samples. N2 adsorption-desorption isotherms and corresponding

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pore size distributions of samples PC-800 and GC-800 are presented in Fig. 5. The isotherms of the sample PC-800 can be defined as type II curve (Fig. 5(A)) [37, 39], which usually represents macroporous (or non-porous) adsorbent and

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unstinted monolayer-multilayer adsorption [40]. The results indicate that the

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sample PC-800 maintains the macroporous structure of the resin CH-90 during the carbonization. A broad hysteresis loop is observed above the relative pressure P/P0 = 0.10, implying a wide pore size distribution just as shown in Fig. 5(A) inset. Its BET specific surface area is calculated at 8.0 m2/g.

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The sample GC-800 shows type IV overlapping type II curves (Fig. 5(B)), implying hierarchically porous structure. The steep N2 uptake below P/P0 = 0.02 coexists with an obvious hysteresis loop of type H3 in the relative pressure range

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from 0.45 to 0.8, which indicate the coexistence of micropores and mesopores

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[41]. The results imply that the emerging mesopores contribute from the removal of Ni nanoparticle. On the other hand, the increasing N2 uptake at a P/P0 = 0.9 ‒ 1.0 can be attributed to the presence of macropores [37]. Its pore size is centered at about 2.2 nm shown in Fig. 5(B) inset. The specific surface area of the sample GC-800 is calculated at 324.4 m2/g. The ratio of pore volume derived from micropore to mesopore is calculated at 1:2, implying a well matched micro/mesoporosity for expected capacitance performance [42]. The 13

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results demonstrate that the pore volume and specific surface area of the sample GC-800 are drastically improved during the Ni involved carbonization. 3.6. Electrochemical performance

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First in the triple-electrode system, the CV curves of the samples PC-800 and GC-800 are collected at the scan rate of 20.0 mV/s in the electrolyte 1.0 M H2SO4 (Fig. 6(A)). Both of them present quasi-rectangular shapes in the

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potential window, indicating an ideally capacitive property. Remarkably, the

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area encircled by the curve from the sample GC-800 is almost triple larger than that from the sample PC-800, implying a higher specific capacitance. The enhancement in specific capacitance can be primarily ascribed to its improved specific surface area and electric conductivity plus fit micro-/mesostructure. CV

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curves of the sample GC-800 are also collected at different scan rates varying from 5.0 to 200.0 mV/s (Fig. 6(B)). As expected, although the current densities increases with the scan rates, the curves still retain rectangular shape even at the

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high scan rate of 200.0 mV/s, displaying a good high-rate performance [41]. It is

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worth mentioning that the specific capacitance of the sample GC-800 is 232.1 F/g and the graphite paper is about 2.0 F/g at the same scan rate of 5.0 mV/s (Fig. S1). The results imply that the graphite paper contributes negligible specific capacitance to the whole electrode. GCD curves of the samples GC-800 and PC-800 at the current density of 0.5 A/g obviously exhibit symmetrical profiles with a triangular shape (Fig. 6(C)), proving an ideal EDLC behaviour. The specific capacitance is 292.8 F/g for the 14

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sample GC-800, which is much higher than that (109.4 F/g) for the sample PC-800. The results can be ascribed to their different structure and texture. GCD profiles for the sample GC-800 are also performed at current densities from 1.0

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to 2.0, 5.0, 10.0, 20.0 A/g (Fig. 6(D)). Similarly, they exhibit quasi-triangular shapes. The specific capacitances are calculated at 261.7, 237.9, 209.7, 182.0, and 151.0 F/g and plotted in Fig. 6(E). The result is superior to the reported

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Cr-catalyzed carbon [26], carbon aerogel [1], and hierarchical porous carbon

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nanofiber [43]. Meanwhile, it is noteworthy to be mentioned that the specific capacitance retention is 89.4%, 81.2%, 71.6%, 62.2%, and 51.6%, showing a better rate performance than reported carbon-base capacitors [25, 44]. For comparison, the specific capacitance of the sample PC-800 is also calculated at

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124.1, 109.3, 91.0, 63.8, 44.9, and 29.7 F/g at corresponding current density (Fig. 6(E)). To explain the relatively high specific capacitance deriving from the sample PC-800 with a low specific surface area, small molecule CO2 is used as

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the probe to detect the existence of possible micropore with diameter smaller

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than N2 molecule. The adsorption isotherms indicate that CO2 adsorption capacity is about 6.5 times larger than N2 adsorption capacity at the same temperature of 0 °C (Fig. S2). Basing on the result, we can conclude that most of micropore in the sample PC-800 is accessible for CO2 molecule but not N2. Also, the micropore is available for the electrolyte ions to form electric double-layer. The EIS analysis is crucial method for explaining the resistivity and efficient charge transfer between the electrode and the electrolyte. The Nyquist plots of 15

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the samples GC-800 and PC-800 are presented in Fig. 6(F). As expected, both samples display almost vertical lines in the low frequency region, which can be attributed to the ideal capacitive behaviour. In addition, the EIS plot of the

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sample PC-800 shows a smaller slope than that of GC-800, which is the typical feature of the a-C and agrees with the previous result [45]. In the high frequency region (Fig. 6(F), inset), a small but clear arc indicates that the sample GC-800

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shows lower charge-transfer resistance and faster ion transportation inside the

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pores. The intercept with the real axis from Nyquist plot is the solution resistance (RS), which includes the sum resistances of the ionic resistance of electrolyte, the intrinsic resistance of the active materials and the interface resistance between the carbon and current collector [46, 47]. The RS values of

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the samples GC-800 and PC-800 are calculated at 0.77 and 1.03 Ω, respectively. Obviously, the sample GC-800 possesses a lower electric resistance than PC-800, which may contribute from its graphitic texture.

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We also prepare samples GC-700 and GC-900 and mainly compare their

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electrochemical performance in the three-electrode system. It is found that they present quasi-rectangular shape-like CV curves with typical EDLC characteristic. But the encircled area of the sample GC-700 or GC-900 is obviously smaller than that of the sample GC-800 (Fig. S3a). Observing from the GCD plots collected at the current density of 0.5 A/g (Fig. S3b), they present quasi-triangular shapes, indicating a good electrochemical reversibility and columbic efficiency. They also show a shorter discharging time than the sample 16

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GC-800. Their specific capacitance is calculated at 264.1 and 208.0 F/g, respectively. Obviously, their specific capacitances are smaller than that (292.8 F/g) of the sample GC-800. The results confirm that the sample GC-800 is

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optimum for the electrode material of supercapacitors. Furthermore, to obtain an insight into specific capacitance at increased current densities, their specific capacitances at corresponding current densities were summarized in Fig. S3c.

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This result is also in accordance with the results aforementioned.

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The CV and GCD curves of the samples PC-800 and GC-800 are also collected in a double-electrode system. Their CV curves are collected at the scan rate of 20.0 mV/s (Fig. 7 (A)). They exhibit almost rectangular shapes and demonstrate a good EDLC characteristic. The area encircled by the curve of the

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sample GC-800 is still larger than that of the sample PC-800, displaying an attractive specific capacitance and agreeing with the above result. Their symmetrical GCD curves at the current density of 0.5 A/g with standard

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triangular shapes also show a good EDLC behavior (Fig. 7(B)). The specific

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capacitance of the sample GC-800 is calculated at 214.0 F/g while that of the sample PC-800 is 78.7 F/g. As shown in Fig. 7(C), the GCD curves of the sample GC-800 display real triangular shapes at various current densities. Its specific capacitances are 205.8, 192.1, 160.9, 126.6 and 92.4 F/g at the current densities of 1.0, 2.0, 5.0, 10.0 and 20.0 A/g, respectively. For careful comparison, the specific capacitances of both samples are also summarized in Fig. 7(D), which are similar to the results obtained in the three-electrode system. It is 17

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common that the specific capacitances in the double-electrode system are lower than them in the triple-electrode system. The result might be ascribed to the prominent difference between the two systems [48].

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No doubt can be imposed on the importance of high specific surface area to the EDLC performance. However, it has been demonstrated that carbon with high specific surface area does not always produce an expected specific

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capacitance [14]. Besides electric conductivity, surface property, pore shape,

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hierarchical pore system can be crucial for final specific capacitance. But the utilization of specific surface area may dominate the contribution to the total capacitance. Taking the specific surface area into consideration, the specific surface-area capacitance of carbon material GC-800 is 66.0 µF/cm2 (Table 1). As

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far as we know, the specific capacitance is not only higher than those from simplex microporous or mesoporous materials but also hierarchically porous ones (Table 1). As concluded from the above results, the sample GC-800 shows

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a specific surface area of 324.4 m2/g and the ratio of pore volume deriving from

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micropore to mesopore of 1:2. The results highlight the importance of effective abutment between micropores and mesopores as well as reasonable configuration on the micro/mesoporosity to the specific capacitance. To further evaluate the performance of the carbon material in the double-electrode system, the energy density of the full cell is also calculated. Based on the equations (4) and (5), the energy density of the full cell basing on the sample GC-800 can be calculated around 7.43 Wh/kg, which is much higher 18

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than that basing on the sample PC-800 (3.06 W/kg) at the same power density of 125 W/kg. Moreover, the sample GC-800 performs higher energy density than the reported carbons. All of them derived from typical carbons are plotted in Fig.

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8 for further comparison, such as hierarchically porous carbon (4.5 Wh/kg at 200.0 W/kg) [33], 3D hexaporous carbon from single-layer graphene (5.3 Wh/kg at 125.0 W/kg) [60], nanoporous nitrogen-enriched carbon (4.4 Wh/kg at 2500.0

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W/kg) [61], PVPox carbon (4.0 Wh/kg at 200.0 W/kg ) [52], porous carbon

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nanofiber (3.22 Wh/kg at 600.0 W/kg) [62], activated carbon (4.2 Wh/kg at 1500.0 W/kg) [63], mesoporous carbon nanofiber (5.1 Wh/kg at 242.2 W/kg) [64], mesoporous carbon intercalating with graphene (2.4 Wh/kg at 6000.0 W/kg) [53]. Interestingly, the full cell based on the sample GC-800 can perform high

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energy density even at high power density, which can be ascribed to its micropores jointing with mesopores.

For practical application, good cyclic stability sometimes is more

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important than the high specific capacitance for supercapacitors. Herein, the

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cyclic stability of the sample GC-800 is firstly evaluated using GCD measurement at the current density of 5.0 A/g in the triple-electrode system (Fig. 9). Its specific capacitance seems frozen at around 200.0 F/g. No capacity attenuation is observed during the 10000 cycles test. Instead, it gains 10% after the measurement, implying a robust pore system in the sample GC-800. The result could be attributed to the carbon being further activated for charge transfer across the surface and interface during the measurement [54, 65, 66]. It can be 19

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obviously seen that the sample GC-800 exhibits an exciting cyclic stability, which can recur in the double-electrode system (Fig. S4). From all the results mentioned above, it is speculated that well jointed micropores and mesopores

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can promise a fast pathway for ion diffusion. Thus, the sample GC-800 is a competitive candidate in supercapacitor applications basing on the high rate capability, large specific capacitance, and everlasting cyclic stability.

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Besides this, we also ascertain the cyclic stability in the two-electrode

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system via the floating technique [67, 68]. The symmetric system was subjected to floating at 1.0 V at the current density of 5.0 A/g, followed by a 10 h voltage holding which is trailed by three GCD cycles to detect any loss of the electrodes on the cyclic life over 400 h [67-70]. As shown in the Fig. 10, the specific

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capacitance gradually gains about 5% during the first 100 h comparing with the initial specific capacitance (153.8 F/g). The result is in accordance with the result of the above GCD measurement, implying a fairly attractive stability [71].

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Similarly, the results can be attributed to the improved wettability or activation

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during the measurement. Although the specific capacitance stepwise shows about 5% decline in next 60 h, it subsequently keeps a constant even after 400 h. In total, the electrode basing on the sample GC-800 displays 95% specific capacitance retention, further indicating an excellent cyclic stability. Taking the widely acceptable criteria to measure end-of-life for the supercapacitor (20% or 30% specific capacitance decay) into consideration [67-70], our results prove that the carbon GC-800 is very promising and potential for electrochemical 20

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supercapacitor. 4. Conclusions In summary, we have prepared graphitic porous carbon by directly

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carbonizing Ni(II) cation-exchanged resin at a moderate temperature. The homogeneously dispersed Ni nanoparticle not only acts as the catalyst for the graphitization of the commercial resin but also participate in fabricating matched structure.

The

graphitic

carbon

GC-800

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micro-/mesopore

exhibits

a

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hierarchically porous structure with specific surface area of 324.4 m2/g. In the double-electrode system, the sample GC-800 display high energy density of 7.43 Wh/kg at a power density of 125.0 W/kg and a specific capacitance of 66.0 µF/cm2 (214 F/g) at the current density of 0.5 A/g. Furthermore, low resistance

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building on the graphitic texture, good rate capability basing on the balanced micro/mesoporosity and remarkable cyclic stability deriving from the robust

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structure make the carbon GC-800 a promising candidate for EDLC application.

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Acknowledgements

The authors acknowledge the financial supports from China Scholarship Council (No. 201506935028), National Natural Science Foundation of China (No. 21476156, 21776191), Key Scientific and Technological Projects of Shanxi Province (No. MD2014-09), and Youth Foundation of Taiyuan University of Technology (No. 2015MS015).

21

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nitrogen co-doped carbon microspheres, Nature Communications 6 (2015) 8503.

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Figure captions Scheme 1. A proposed mechanism of graphitic carbon with balanced porosity derived from monodispersed nickel nanoparticle containing cation-exchange

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resin. Fig. 1 SEM images of the samples: (A), (B) PC-800, and (C), (D), (E), (F) GC-800.

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Fig. 2. XRD patterns of the samples: (A) PC-800, (B) Ni-GC-800, and (C)

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GC-800.

Fig. 3. Raman spectra of the samples: (A) PC-800, and (B) GC-800. Fig. 4 TG (A) and DTG (B) curves for the samples: (a) PC-800, (b) GC-800, and (c) Ni-GC-800.

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Fig. 5. N2 adsorption-desorption isotherms of the samples: (A) PC-800, and (B) GC-800. The insets display their corresponding pore size distribution. Fig. 6. (A) CV curves of the samples PC-800 and GC-800 at the scan rate of

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20.0 mV/s, (B) CV curves of the sample GC-800 at scan rates from 5 to 200

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mV/s, (C) GCD curves of the samples PC-800 and GC-800 at a current density of 0.5 A/g, (D) GCD curves of the sample GC-800 at current density from 1.0 to 20.0 A/g, (E) specific capacitance of the samples PC-800 and GC-800 at current density from 0.5 to 20.0 A/g, and (F) Nyquist plots derived from the samples PC-800 and GC-800. Fig. 7 (A) CV curves of the samples PC-800 and GC-800 at the scan rate of 20.0 mV/s, (B) GCD curves of the samples PC-800 and GC-800 at the current density 33

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of 0.5 A/g, (C) GCD curves of the sample GC-800 at different current densities, and (D) specific capacitance of the samples PC-800 and GC-800 at different current densities.

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Fig. 8. Ragone plots of the samples GC-800, PC-800, and reported materials in the double-electrode system

Fig. 9. Cyclic stability of the sample GC-800 at the current density of 5.0 A/g in

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the triple-electrode system

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Fig. 10 Cyclic stability of the device based on the sample GC-800 at 1.0 V and

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the current density of 5.0 A/g via the floating technique.

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ACCEPTED MANUSCRIPT Table 1. Specific capacitances in double electrode system of the sample GC-800 and some typical carbon materials reported in the literatures Electrolyte

Cm (F/g)

Scan rate

Cs (uF/cm2)

Cv (F/cm3)

Work

GC-800

1.0 M H2SO4

214.0

0.5 A/g

66.0

552.6

Herein

1.0 M H2SO4

205.8

1.0 A/g

63.5

531.5

Herein

NPC-PAN800

1.0 M H2SO4

191

1.0 A/g

30.0

NC-3

1.0 M H2SO4

183

0.23 A/g

18.3

CP2

4.0 M H2SO4

290

0.001 A/g

13.0

CS48

1.0 M H2SO4

202

0.5 A/g

10.1

CDC-Aero-700

1.0 M H2SO4

151

0.1 A/g

GMCS-NH3

6.0 M KOH

29.6

0.1 A/g

TC-1

6.0 M KOH

54.4

Cr-treated carbon

6.0 M KOH

122.5

MCS

6.0 M KOH

225

HPCF

6.0 M KOH

329

3D HPG

6.0 M KOH

305

Ref.[49]

207.9

Ref.[50]

467.7

Ref.[51]

348.7

Ref.[52]

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122.4

65.6

Ref.[1]

2.13

9.0

Ref.[53]

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7.22

0.5 A/g

2.57

44.6

Ref.[54]

1.0 A/g

5.79

N/A

Ref.[26]

0.25 A/g

9.39

77.3

Ref.[55]

0.1 A/g

15.1

258.6

Ref.[56]

0.5 A/g

16.8

250.0

Ref.[57]

TE D EP AC C

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Carbon material

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EP

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Highlights 1. Nanosized graphitic carbon with balanced and interconnected

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micro-/mesopore is made. 2. Monodispersed Ni nanoparticles catalyze the graphitization and the formation of pores.

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volumetric capacitance of 552.6 F/cm3

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3. GC-800 delivers a specific capacitance of 66.0 µF/cm2 and a

4. It displays a high energy density of 7.43 Wh/kg at a power density of 125.0 W/kg in symmetry cell.

5. It shows good cyclic stability after 400 h test in double-electrode

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system via the floating technique.